Every once in a great while it's possible to find a 3rd gen 9mm pistol where the extractor might fit in a slide and function normally without filing. Not very common at all, but the tolerances on the new parts are much better. The problem comes with the looser tolerances of the older production 3rd gen guns.
Also, sometimes in the older slides the spring hole might be out of spec by either depth or the shape of the bottom of the hole (which could change the way the bottom coil fit in the hole, and therefore the way the spring would sit in the hole). I was told by the repair center that a properly sized hand-powered ball end mill tool was sometimes required to correct the problem, although the nested spring set used for a while for repair of 59XX series guns worked well, too.
On the older models with the narrow barrel tab & breech face, the tension of the extractor spring should be 4-8lbs on the force dial gauge when extractor tail movement is detected (when the dial gauge hook pulls the extractor hook). On the newer models with the wide barrel tab & breech face it should be somewhere between 7-11lbs.
If an armorer can't get the proper spring weight to be met, the gun should be returned to S&W. It might require some further attention that isn't commonly taught in the armorer classes. I've encountered some instances where a particular gun required a heavier tension on the extractor for normal extraction/ejection, and made the repair myself after discussing it with the factory repair center techs, but care had to be taken to ensure the increased tension didn't reach the point where it interfered with proper feeding.
A local gunsmith familiar with working on S&W pistols might be able to make the repair, you know.
